Key Questions to Ask When Ordering Dry bulk storage silos

In liquid and dry bulk storage, quality and value drives today's storage tank selection process. With advances in fabrication technology, engineering design, coating processes and field construction techniques, some storage products utilized in the past have become outdated, while other products have pushed to the front of the line in product development and field performance.

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The process of selecting a liquid storage tank or a dry bulk storage silo starts with a series of questions that require answers before proceeding. This is one of the main problems witnessed in today's containment and storage markets. Many times storage tank selection is based on a preferred construction type in lieu of stored product performance requirements. Tank manufacturers tend to process customer information relative to their standard storage products and design parameters, which leaves the client responsible for the outcome. This is an archaic approach which is safe for the vendor, but often misses the mark in achieving a functional and efficient storage tank system that performs per your requirements.

The "top 10" list below includes basic considerations that should be addressed on every storage application. If the application under review is routine and no problem areas exist with the product/liquid stored, a standard storage tank quotation can be generated. If any items listed are a concern, a specialist in storage applications should advise you on the correct approach.

CONSIDERATIONS — DRY BULK SILOS

• How much material will be stored in tons/cubic feet?
• Are there any unique characteristics of the stored material to consider?
• What materials of construction should be used?
• What product density should be used for volume and design calculation?
• Is material degradation a concern?
• Is material segregation a concern?
• What type of material discharge pattern is preferred or required for the application? (funnel flow, mass flow, expanded flow, etc.)
• Is a hopper flow aid device required for reliable discharge?
• Should my material be tested by a “flow specialist?”
• Do I need a reliable performance guarantee from the silo supplier?

CONSIDERATIONS — LIQUID STORAGE TANKS

• Capacity requirements in gallons/cubic meters?
• Project design conditions (specific gravity, wind, seismic, snow loads, etc.)?
• Are there any unique "storage process" requirements (agitation, digester, pressure/vac., etc.)?
• pH and temperature of stored liquid?
• What materials of construction should be used?
• System interface responsibility?
• Configuration type (flat bottom, sloped bottom, elevated, etc.)?
• Corrosion allowance and/or cathodic protection requirements?
• Foundation requirements (concrete ring wall with compacted fill, base setting ring, concrete slab, steel bottom with external saddles, etc.)?
• Specific design code (AWWA, API 650, NFPA, FM, AISC, NSF, etc.) requirements?

You don't need to have all the answers, but you do need to know a reliable storage/containment specialist that can advise you in these areas. With the basics covered, the selection process can be narrowed for customer review and preference.

Example 1:

Liquid storage application requires 2 million gallons of wastewater storage. The PH of stored wastewater is relatively neutral. Tank construction evaluation is narrowed down to field-welded storage tank , concrete storage tank, and bolted storage tank designs. Bolted tank design is selected due to recognized field performance of non-leak design and factory applied powder coat system. Based on system configuration, two tanks are utilized.

Example 2:

We need to store cubic feet of hydrated lime. Based on the capacity requirements as detailed below, a bolted storage tank or shop-welded storage tank is correct for the application. Based on experience with the material and an extensive number of applications in the field, a flow aid device is required. A bin activator sized at half the tank diameter will provide reliable material discharge. A bin activator provides a funnel flow material discharge pattern. The material does not need to be tested. Material degradation should not be a concern.

Example 3:

Potable water storage tank application requires 1 million gallons of containment. A condensed field construction schedule is critical to the project. AWWA specification requirements are applicable. Tank selection is narrowed to bolted storage tank and field-welded storage tank construction. Based on expedited schedule requirements and superior performance of factory applied powder coat systems available for potable water storage, bolted storage tank construction is selected. International location — order to installation complete within 10-12 weeks.

Example 4:

Storage application requires 25,000 cubic feet of a granular product. It is desired to minimize material segregation during discharge. First we will note that size selection exceeds a shop-welded tank. This eliminates the shop-weld tank, unless we break the capacity requirements into a two tank package. The correct storage selection is a bolted smooth-wall storage tank. The correct flow discharge selected is mass flow. This does not mean we specify a silo designed for mass flow design loads. The correct request is to specify a storage silo designed for functional mass flow discharge. In order to provide reliable "functional mass flow" (FIFO) discharge, the material will need to be tested.

In general, storage tank applications that require a storage volume of less than 10,000 cubic feet (dry) or 30,000 gallons (liquid) will utilize a shop-welded or smoothwall bolted storage tank. Both products are used interchangeably in the industry. Shop-welded storage tank and bolted storage tank construction maintain advantages as listed above. Under review, the total installed costs (material + freight + field installation) are comparable between both products.

Bolted storage tanks, field-welded storage tanks, concrete storage tanks and hybrid storage tanks are typically utilized in large volume storage applications. All four designs are routinely specified in the power industry. In the industrial market, bolted tank and hybrid tank construction are the most cost efficient storage silo designs. Field-welded tank and concrete storage tank construction require extended field installation timeframes, which equates to higher installed cost.

Relative to "materials of construction" selection, bolted storage tanks, shop-welded storage tanks, and field-weld storage silos are available in coated carbon steel, stainless steel and aluminum construction. All storage tanks and storage silos are customized for the application and are available for installation at grade level or elevated. Typical storage tank accessories include manway accesses, level control nozzles, maintenance access platforms, agitators, heaters, insulation, caged ladders, spiral stairways, perimeter guardrails, and custom requirements for system integration. In summary, if the basics are covered in storage tank selection, years of trouble free containment can be expected. In today's industrial and municipal storage markets, a storage specialist should advise or confirm your selection. In all cases, a reliable storage tank system starts with a properly designed and configured tank/silo. Guesswork has been removed from the equation.

Published in the November issue of Powder & Bulk Engineering

When selecting a new bulk solids silo, determining your application’s requirements in advance can help the supplier provide an efficient, cost-effective design. This article will help you identify the storage needs to include in a request for proposal to a silo supplier.

Just because your company handles bulk solid material doesn’t mean you have to be an expert in bulk solids storage equipment. While being aware of preferred solutions on the market is helpful, you don’t have to wade through endless websites, sales brochures, and technical literature to determine the best storage solution for your application. To reduce headaches and save time and money, create a list of your application’s specific requirements before you engage a supplier. Having a clear idea of what you want to accomplish will help you ask the right questions and manage your spending judiciously.

Whether you handle food, frac sand, minerals and aggregates, chemicals, plastics, or petrochemicals, an off-the-shelf silo is unlikely to be your ideal storage solution. In addition to a wide variety of silo support options, including standard or short skirted, leg, or lug supports, your silo will likely require additional components such as dust collector flanges, manways, nozzles, flow control or internal blending devices, stairways, peripheral conveyor supports, or interior maintenance platforms. With so many potential add-ons, you should look for a supplier that can provide a variety of adaptable systems that integrate seamlessly with these components.

Your silo should be designed based on your specific application requirements, including the site location; seismic, wind, and snow loads; the material being stored; the material feed and discharge methods; how the material is being used; other equipment in use; time constraints; and your budget. When preparing your list of requirements, be sure to consider the following:

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Material being stored

Let’s start with the type and amount of material to be stored because that will dictate the silo type and the necessary features. Your supplier will need to know the material’s characteristics, including particle size (such as fine powder or granular); hygroscopicity; abrasiveness; and compacted bulk density in lbs/ft3.

Consider the volume of material your application handles and make sure the silo can be built to the exact dimensions you need. Dry bulk storage silos are typically made of shop-welded steel, field-bolted steel, or concrete. (Field-welded steel silos are also available but are most commonly used for liquid applications.)

A Shop-welded steel silo is suitable for plastic resins, food products, dry chemicals, minerals, wood waste, and other miscellaneous dry products. The silo is delivered to the site in one piece, which is ideal for handling but limits the silo’s length and diameter. Maximum capacity is typically about 10,000 cubic feet, but additional storage capacity can be added by splicing the skirt to extend the silo’s overall height to greater than 140 feet.

A field-bolted steel silo is erected in place from steel panels, so it can be larger than a shop-welded steel silo, with a diameter commonly ranging from 15 feet up to 50 feet with a hopper and a capacity of up to 225,000 cubic feet. Shipping cost is lower than for a shop-welded silo, and the steel panels can be powder coated at the factory rather than painted on-site, so installation can be 2 to 3 times faster than for a shop-welded silo. A field-bolted silo can be designed to store a range of particle sizes (from fine dust to large chunks) and bulk densities (exceeding 140 lbs/ft3 in some cases). Common applications include cement, frac sand, fly ash, coal, limestone, lime, aggregates, minerals, chemicals, plastics, select foods, and wood waste.

A concrete silo is typically very large and is mainly used when the cost of bolted steel is prohibitive. A concrete silo often has a flat bottom rather than a sloped hopper bottom and may use an internal reclaimer (a screw that spins as it revolves around the center of the flat silo floor) to draw poorly flowing material into the discharge. Concrete silos are often used for storing large biomass materials, such as wood chips or fuel for alternative-fuel power plants.

How you intend to feed material to the silo and discharge material from the silo will help determine the most cost-effective size. You can sometimes save money by building a taller and narrower silo, effectively reducing the stored material’s overall downward force and minimizing stress on the silo’s hopper support.

If your material is resistant to flow or frequently hangs up, causing obstructions, make sure the silo’s interior surface is smooth and corrosion resistant. Some silos feature a baked-on, dry powder coating for smooth material flow.

Also consider whether your material’s composition, consistency, or bulk density are such that your silo will require flow-aid devices such as bin activators, vibrators, aeration, or mechanical agitation to promote flow and prevent bridging or ratholing.

Discharge Height

Consider the silo’s discharge height carefully, as this is a cost driver. For example, if the silo will be used for loading trucks, you may need a loading spout (either dustless or not) and possibly a positioner. Dustless loadout requires the addition of an integral dust collector, which can add up to 3 feet to the silo’s discharge height. A positioner can add 2 to 3 more feet depending on the unit, but this addition will increase loadout speed and reduce operation costs. If the silo is for storing incoming materials, consider how much space you’ll require below the discharge to install and maintain the equipment for conveying the material from the silo to your process.

The type of flow desired will primarily dictate the silo’s hopper slope, which also influences the discharge height. Mass-flow (first-in first-out) generally requires a steeper hopper slope than funnel flow (first-in last-out). As a result, a mass-flow hopper will likely be taller than a funnel-flow hopper, which will reduce the clearance below the discharge. You should also consider the hopper’s outlet diameter, which will affect the discharge flowrate.

Site location

The size of the area where the silo will be located will help determine the silo type and construction method. Your installation approach will also vary depending on whether the silo location is a greenfield site (new construction) or a brownfield site (adding to an existing facility). A greenfield site is a blank slate; you have a clear foundation to build on and there’s no other equipment around. When installing a silo next to existing equipment on a brownfield site, you can’t afford down time and may have tight quarters with height restrictions or equipment overhead. Make sure the supplier knows about your space restrictions and is capable of installing the silo in the area available. Accommodating a crane requires a great deal of space and sometimes great expense or interruption to your process, while a jacking system or scaffold will exceed the silo’s footprint by only a few feet.

Field-bolted silos typically come with either flat or chimed panels. Flat panels overlap vertically on all four edges, while a chimed panel has horizontal flanges along the top and bottom edges so panels fit together and form a compression seal using synthetic rubber gaskets. Chimed panels are typically assembled from the ground up using a scaffold, but flat panels are commonly assembled from the top down using a jacking system. Workers bolt each successive row of panels together at ground level and then jack up the assembled portion before attaching the next row below it. Working at ground level makes attaching components to the panels much easier compared to lifting them into place at elevation, but jack rental and other required equipment costs may outweigh the cost of additional time required to erect scaffolding. With either method, be sure to ask your potential supplier about safety systems provided and the supplier’s EMR (emergency modification rating, used by the insurance industry to measure a company’s past injury claims and potential risk). Also, confirm that the supplier’s installation methods have been approved by a third-party, professional engineer with experience in safety procedures.

If you already have a conveyor on the silo pad, building a scaffold around and above the conveyor is more effective than using a jacking system. However, if you have an existing overhead conveyor or other obstruction preventing you from bringing in a crane to lift the silo roof into place, using a jacking system to assemble a flat-panel silo at grade is likely the better solution.

Construction Time

Labor will be a significant chunk of the project cost. One of the main advantages of a flat-panel, field-bolted silo is the speed of construction using the jack installation method. This is particularly beneficial for installations that include multiple or very large silos. For example, if you’re building four silos and you’re saving a week per silo using jack installation versus scaffold installation, you’ll complete the installation a month sooner and significantly reduce labor costs. A large silo can be jack built in 20 to 25 days versus 45 to 60 using a scaffold process.

Maintenance Requirements

You should also consider what you’ll need to do to maintain high performance at minimal cost throughout the life of the silo. Make sure your silo has easy access to internal components for routine maintenance, repair, or replacement. A bolted silo uses either gaskets, sealants, or a combination of the two to seal the joints between panels, and resealing joints is another maintenance expense. When comparing silos, you should understand the properties of the gaskets or sealants used and consider the quality and testing of those materials. And since a large bolted silo may have tens of thousands of bolts, you should also ensure that the bolts used on the silo you select have the necessary strength and corrosion resistance for your application.

Finally, don’t forget to consider the silo’s appearance, especially for silos that are visible to the public. Painted silos need repainting, at great cost, while powder coatings are permanent and resist corrosion and weathering. Not all factory-applied, heat-cured coatings are created equal, however. When selecting a supplier, ask about the coating application specifications. Adhering to the coating’s application specifications is the most important factor in determining the coating’s longevity, but any factory-applied coating will typically provide better overall longevity than a coating that’s applied in the field. Also ask if the supplier tests for coating holidays (defects such as pinholes, cracks, or insufficient bonding) and, if so, using which method, according to which standard, and how frequently. Some standards only call for random visual inspection while others are more comprehensive. A 67-volt wet sponge tester, for example, signals an alarm when the tester locates a coating holiday, but a 1,100-volt tester signals an alarm and then burns through the coating to mark the holiday’s location and ensure that the defective area is recoated.

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